System and method for precision acoustic event detection

ABSTRACT

A system and method for providing precision locations for sensors which make up an array of sensors in a gunshot detection system. In a preferred embodiment sensors employ a commercial GPS which reports a sensor position or a group of pseudoranges to GPS satellites. A server collects differential information from a differential node and, in one preferred embodiment, calculates a precision position for each sensor by adjusting the reported position or pseudoranges with the differential information. In another preferred embodiment differential information is sent from the host to individual sensors which calculate their own precision positions. Differential information may be latitude and longitude corrections, pseudorange corrections, ionospheric delay, GPS satellite clock drift, or other corrective term which will improve the accuracy of a sensor position.

This application hereby claims priority back to U.S. ProvisionalApplication Ser. No. 60/481,915, filed on Jan. 19, 2004, herebyincorporated by reference as if set forth fully herein.

BACKGROUND OF THE INVENTION

The present invention relates to a system and method for detecting andlocating an acoustic event. More particularly, but not by way oflimitation, in a system for identifying and locating an acoustic event,the present invention provides a system and method for achieving highlyaccurate position information for individual sensors.

Gunfire and sniper detection systems are generally known in the art.Such systems can be broadly grouped into three categories: systems whichpinpoint the precise location of the source of gunfire; azimuthalsensors which provide an indication of the radial direction to thesource of gunfire; and proximity sensors which merely provide anindication that nearby gunfire was detected. While such systems havebeen demonstrated to perform well in both law enforcement and militaryapplications, the entire field is presently an emerging technology.

In many large cities, gun-related violence has become a plague ofepidemic proportions. Urban gunfire, whether crime-related orcelebratory in nature, results in thousands of deaths per year in theUnited States alone. Gunfire location systems, such as those installedin the Redwood City, Calif., Glendale, Ariz., Willowbrook, Calif., Cityof Industry, Calif., and Charleston, S.C. areas, have proven to beeffective in reducing law enforcement response time to detected gunfire,apprehending criminals, collecting evidence, and reducing the occurrenceof celebratory gunfire. One such system is described in U.S. Pat. No.5,973,998, issued to Showen, et al., which is incorporated herein byreference.

Showen, et al. discloses a system wherein sensors are placed at adensity of roughly six to ten sensors per square mile. Audio informationis sent to a computer at a central location and processed to: detect agunshot; determine a time of arrival for the gunshot at each sensor; andcalculate a location of the shooter from the differences in the times ofarrival at three or more sensors. Showen, et al. takes advantage of thelong propagation distance of gunfire to place sensors in a relativelysparse array so that only a few of the sensors can detect the gunfire.This permits the processor to ignore impulsive events which only reachone sensor—a concept called “spatial filtering.” This concept of spatialfiltering radically reduces the sensor density compared to predecessorsystems, which require as many as 80 sensors per square mile.

Another gunshot location system is described in co-pending U.S. patentapplication Ser. No. 10/248,511 by Patterson, et al., filed Jan. 24,2003, which is incorporated herein by reference. Patterson, et al.,discloses a system wherein audio information is processed within eachsensor to detect a gunshot and determine a time of arrival at thesensor. Time of arrival information, as determined from a synchronizedclock, is then transmitted wirelessly by each sensor to a computer at acentralized location where a location of the shooter is calculated inthe same manner as in the Showen, et al. system.

As yet, azimuthal systems have not been as widely accepted as, forexample, the Showen, et al. system. Azimuthal sensors typically employone or more closely-spaced sensors, where each sensor includes severalmicrophones arranged in a small geometric array. A radial direction canbe determined by measuring the differences in arrival times at thevarious microphones at a particular sensor. Presently such systemssuffer from somewhat limited accuracy in the determination of the radialangle, which in turn, translates into significant errors in thepositional accuracy when a location is found by finding the intersectionof two or more radial lines, from corresponding sensors, directed towardthe shooter. Since errors in the radial angle result in ever increasingpositional error as the distance from the sensor to the sourceincreases, the reported position will be especially suspect toward theouter limits of the sensors' range.

To provide an absolute location for an event, the location of reportingsensors must be known. In a fixed system, the location of each sensorcan be surveyed at the time the sensors are placed. In a system withmoving or re-locatable sensors, each sensor typically self-surveys witha global positioning system receiver (“GPS”) or other such system. Aswill be appreciated by those skilled in the art, several factors canimpact the accuracy of a location provided by a GPS receiver which, inturn, impacts the accuracy of a source location provided by the gunshotlocation system.

GPS receivers can be broadly divided into two categories, commercial orcivilian receivers and military receivers. Commercial GPS receivers usethe L1 frequency of the GPS signal to acquire the timing informationused to determine position and perhaps the L2 frequency to determineatmospheric delays while military receivers use both the L1 and L2frequencies to determine the position. Encryption keys to decode the L2signal are controlled by the U.S. government and generally restricted tomilitary applications. In general, military GPS receivers are moreaccurate than their commercial counterparts but, for a variety ofreasons, tend to be larger, consume more electrical power, and aredramatically more expensive. In times past, selective availability(“SA”) was employed to further degrade the positional accuracy ofcommercial GPS receivers. However, the U.S. government is now fullycommitted to eliminating SA except regionally at times of conflict orother such threat.

A number of schemes have been developed to improve the accuracy ofcommercial GPS receivers such as: differential GPS (“DGPS”) where anetwork of fixed ground-based reference stations broadcast thedifference between actual pseudoranges and measured pseudoranges; theWide-Area Augmentation System (“WAAS”) which uses a series ofground-based stations operating in concert with a constellation ofgeosynchronous satellites to provide WAAS enabled GPS receivers withinformation such as atmospheric delay, individual satellite clock drift,and the like; Local-Area Augmentation Systems which are WAAS-like innature but transmit the corrective information from ground-basedstations locally, instead of satellites; as well as others. Each systempresently suffers from limitations, such as: DGPS requires a secondreceiver and a nearby ground-based station and DGPS is particularlyuseful for overcoming the effects of SA but is of less value since SA isgenerally no longer active and WAAS has broader coverage; the WAASsystem is limited to North America, requires a clear view of thesouthern sky, and is still in deployment such that presently not allareas enjoy reliable WAAS augmentation; and LAAS systems will have verylimited coverage, strictly near major airports and require specializedreceivers.

As it relates to gunshot detection systems, the accuracy of commercialGPS receivers is somewhat limiting. Unfortunately, military GPSreceivers are generally not available for law enforcement applicationsand thus, such acoustic sensors are limited by the characteristics ofcommercial receivers. Even in military applications, withbattery-operated gunshot detection sensors, such as soldier-wornsensors, the size, weight, cost, and electrical power consumption ofmilitary GPS receivers presently tips the balance towards using acommercial version, despite accuracy concerns.

It is thus an object of the present invention to provide a system andmethod for improving the positional accuracy of an array ofself-surveying acoustic sensors which incorporate commercial GPSreceivers.

SUMMARY OF THE INVENTION

The present invention provides a system and method for improving thepositional accuracy of acoustic sensors in a gunshot detection system.In a preferred embodiment, the inventive system includes: a plurality ofacoustic sensors dispersed throughout a monitored area, each sensorhaving a commercial GPS receiver; a host processor at a known location,the host processor also having a commercial GPS receiver; and acommunication network adapted to deliver information from the sensors toa host processor. Since the location of the host is known, errors in theGPS provided position at the host are used to correct the positionalinformation provided by each sensor to improve the accuracy of thereported position of the source of an event.

In one preferred embodiment, the host is located at a fixed, surveyedposition. Unlike DGPS and WAAS, no special receiver is required sincethe correction for all reporting sensors is performed at a centralizedlocation.

In another preferred embodiment, the host further includes a militaryGPS receiver, or similar highly accurate positioning system, thus makingthe host portable or re-locatable. Since the position of the host isalways known with a high degree of accuracy, differences between thereported positions of the commercial receiver and the military receivercan be determined and applied to the positions of reporting sensors.

Further objects, features, and advantages of the present invention willbe apparent to those skilled in the art upon examining the accompanyingdrawings and upon reading the following description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a preferred configuration of the inventive gunshotdetection system.

FIG. 2 provides a perspective view of a preferred embodiment of a sensoras employed in the inventive system.

FIG. 3 provides a block diagram of a preferred embodiment of the sensor.

FIG. 4 provides a block diagram for a preferred embodiment of the hostnode of the inventive system.

FIG. 5 provides a block diagram for a preferred embodiment of theinventive system having a dual GPS node independent of the host.

FIG. 6 provides a diagram showing the relationship of the inventivesystem to the GPS and WAAS constellations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the present invention in detail, it is important tounderstand that the invention is not limited in its application to thedetails of the construction illustrated and the steps described herein.The invention is capable of other embodiments and of being practiced orcarried out in a variety of ways. It is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and not of limitation.

Referring now to the drawings, wherein like reference numerals indicatethe same parts throughout the several views, a representative gunshotdetection system 100 is shown in its general environment in FIG. 1. In apreferred embodiment, a plurality of sensors 102-106 are dispersed overa monitored area. Preferably, each sensor is placed such that it has arelatively unobstructed acoustic view around its immediate area. By wayof example and not limitation, suitable sites include: placed atop abuilding; placed atop utility or light poles; on towers, etc. Typicallysensors 102-106 communicate through a communication network 108 with acentralized processor 110 wherein information concerning acoustic eventsis processed to provide details of the event, such as the sourcelocation of a gunshot, time of the gunshot, the number of detectedgunshots, the type of event, and the like. It should be noted thatsensors 102-106 may be any combination of wired or wireless sensors,that communications paths 112-116 may carry either analog or digitalsignals, and that network 108 may comprise any combination ofsub-networks, such as, by way of example and not limitation: a telephonenetwork; the internet; a private computer network; a wireless network,or even a collection of dedicated wires routed to the sensor array.

In a military environment it may be preferable that sensors 102-106 areman-wearable. In such a system, the host computer 110 and displaydevices 120 would likely be carried by a squad leader. In such as asystem, other ancillary systems may use data collected by the system andreported in real time, or near real time, to higher levels or commandcenters.

As will be appreciated by those skilled in the art, information about adetected acoustic event is typically output to a person of interest suchas a police dispatcher or directly to individual officers, as throughnetwork 118 to display devices 120 or a computer console. When weapon122 is fired, the muzzle blast reaches sensors 102-106 at differenttimes based on the speed of sound and the distance of each sensor fromthe shooter. Whether the acoustic information is processed at thesensor, or at computer 110, a time of arrival is determined for eachsensor and the differences of the various times of arrival are processedto determine a location of the source of the gunshot. In response to thegunshot, information is provided at device 120.

A preferred embodiment of a sensor 102 is shown in FIG. 2. Typicallysensor 102 includes a housing 200, a support (not shown) for mountingsensor 102, and a windscreen 204 for protecting internal elements fromthe environment, while allowing acoustic waves to pass through to theinterior. If sensor 102 is wireless, antenna 206 is provided for radiofrequency communication.

With reference to FIG. 3, a preferred embodiment of a wireless sensor102 includes: a microphone 210 for receiving acoustic information; anamplifier and/or other signal conditioning 212; a processor 224,typically a digital signal processor (“DSP”), as are well known in theart; a satellite positioning receiver, e.g. a GPS 226 and GPS antenna216; and an interface 214 for communication via a communication network.It should be noted that in such a sensor, GPS receiver 226 may play tworoles, providing positional information as to the sensor's location andan exceptionally accurate real time clock. In one preferred embodiment,DSP 224 includes an analog-to-digital converter 218 to digitize theaudio signal for processing to detect a gunshot and determine the timeof arrival of the gunshot. A sensor suitable for use with the presentinvention is described in co-pending U.S. patent application Ser. No.10/248,511 by Patterson, et al., filed Jan. 24, 2003, which isincorporated by reference hereinabove.

As will appreciated by those skilled in the art, if sensor 102 is manwearable, whether for military or law enforcement applications, size andweight are important considerations. In turn, electrical powerconsumption is likewise of prime concern since it has a direct impact onthe size and weight of batteries required to operate sensor 102.

With presently known gunshot detection systems, it is not possible toprovide an accurate position of the shooter with a single sensor. As aresult, the calculation of a shooter position requires computation basedon the outputs of a plurality of sensors. In a preferred embodiment thesensors communicate with a computer or server 110 as shown in FIG. 4,also referred to herein as a “host node”. Server 110 preferablycomprises: CPU 302; an interface 312 for communication via acommunication network; commercial GPS receiver 304; and GPS antenna 306.If the host node is portable or re-locatable, preferably host node 110further includes military GPS 308, or other precision position locatingsystem. It should be noted that GPS 308 likewise includes an antenna 310although both GPS 304 and 308 can alternatively share a single antenna.

With further reference to FIG. 3, in practice a gunshot is received bymicrophone 210 at three or more sensors 102. At each sensor 102 thereceived audio is amplified by amplifier 212, digitized through A/Dconverter 218, and processed in processor 224 to determine if the soundis indeed a gunshot and, if so, a time of arrival of the event. Aposition is then retrieved from GPS 226 and transmitted via interface214 to the host 110. At the host 110, if three or more sensors reportthe event, the precise positions of the each sensor are determined bycomparing the position reported by receiver 304 with the known positionof host 110 and applying the correction to the reported positions fromeach sensor. After determining precision locations for each sensor, thedifferences in the times of arrivals from the reporting sensors are usedto calculate the source location of the gunshot.

Turning to FIG. 6, while the GPS system is well known in the art, abrief description of the system and it relationship to the presentinvention may be helpful. The GPS constellation, represented bysatellites 504-510, presently consists of 24 satellites in orbits whichare not geosynchronous. Thus the number of satellites, the particularsatellites, and their positions in the sky, relative to a point onearth, is always changing. Above the GPS satellites 504-510, in ageosynchronous orbit, are the WAAS satellites 502. Each GPS satellite504-510 constantly sends, among other things, timing information on itsL1 carrier. Information regarding each satellite and its position inspace, sometimes called the almanac, is broadcast to all GPS receivers,i.e. 520-524, so that each receiver can determine the position of eachsatellite in the sky. Since the receiver, using 522 as an example, knowsthe position of each GPS satellite 504-510 and since it receives timereferenced signals from the GPS satellites, using the differences in thetime of travel of the signal, receiver 522 can determine the distances512-518 to satellites 504-510, respectively. These distances are knownas “pseudoranges.” For receiver 522, finding its position on earth issimply a matter of finding the intersections of the spheres defined by asatellite at the center and having a radius equal to the correspondingpseudorange. It is generally held that a GPS receiver using only the L1frequency can normally find its position in two dimensions within 10meters.

A number of conditions exist which are outside of the control ofreceiver 522 which may affect the accuracy of the calculation. Oneexample is ionospheric delay. As radio frequencies enter the atmosphere,there may be some degree of refraction as the wave strike theionosphere. As can be seen in FIG. 6, the incidence angle between asatellite and the atmosphere, relative to receiver 522, changes with thesatellites position in the sky. The resulting refraction slightlydistorts the path to receiver 522, causing the signal to arrive late.Another condition is GPS clock drift. Since pseudo ranges are determinedbased on time of transmission relative to the speed of light, evenslight inaccuracies in the time clocks between satellites will impactthe position determination.

To overcome such errors, ground stations, represented by station 526,have been established as part of WAAS in the United States and EGNOS inEurope. Each station is at a precise location and, since the preciselocations of the satellites are also known from the almanac, errors inthe pseudoranges can be determined. Once the errors are determined, thecontributing factors are determined, whether from ionospheric delay orclock drift, and the information is up-linked to satellites 502 whichreturn the information to receiver 522. Receiver 522 can then correctits calculated pseudoranges and calculate an improved position. WhenWAAS is available to a WAAS enabled receiver, it is generally held thatthe normal accuracy is about one meter.

GPS satellites 504-510 also transmit information on an L2 frequency.Since L2 and L1 are different, a receiver 512 using both frequencies candetermine the differences in the pseudoranges from each signal anddetermine the degree to which the signals were delayed through theatmosphere.

In one preferred embodiment sensors 102 report pseudoranges along with acurrent position to host 110. At host 110, the pseudoranges are adjustedto reflect atmospheric delay or clock drift based on the pseudorangesreceived at GPS receiver 304 and in light of the known position of host110. Alternatively, atmospheric delay and clock delay values may betransmitted from host 110 to the sensor array such that pseudorangecorrections are applied at each sensor 102 before its position isreported.

In other preferred schemes, the host position does not need to be known.In one such scheme, as shown in FIG. 5, a differential node 400 having amilitary GPS 402, and optionally a commercial GPS 404, is located remotefrom host 406. Differential and atmospheric delay information iscollected at differential node 404 and periodically transmitted to thehost 406 via a network interface 408. Host 406 either uses the receivedinformation to improve the precision of sensors positions or,alternatively, passes the information on to sensors 410 so that sensors410 can calculate and report precision locations. It should be notedthat ionospheric delay could be estimated within GPS 402 withoutresorting to GPS 404.

In another scheme suitable for use where WAAS signals are available,WAAS information is collected by the host, either through its local GPSreceiver, from a differential node, or from any sensor which isreceiving WAAS information. The WAAS information is then either used bythe host to improve the precision of sensor locations or passed on tosensors, which are not receiving WAAS information directly, where theinformation is used to improve the precession of reported locations.

It should be noted that the term “differential node” is used to describeany node, whether also a host or server, a specialized sensor, or adedicated node, which provides differential position, pseudorangecorrection, ionospheric or atmospheric delay, GPS satellite clock drift,WAAS information, or other information used to improve the accuracy of aposition fix provided by a sensor. It should also be noted that whilesensors subject to correction from such differential data should beproximate the differential node, since the GPS satellites are thousandsof miles above earth, proximate distances between sensors and thedifferential node may be in excess of one hundred miles. “Proximate” isused to describe distances where the corrective information providesmeaningful improvement in the calculated position of a sensor.

It should also be noted that while preferred embodiments of theinventive systems were described with reference to a GPS positioningsystem, the invention is not so limited. The inventive system can alsobe used to improve the accuracy of positions obtained from otherpositioning system such as, by way of example and not limitation: GNSS,GLONASS, Galileo, MSAS, Look-Down, LORAN, etc. Similarly, the term“WAAS” should be construed broadly to include any satellite basedaugmentation system, such as EGNOS and the like. Such changes are withinthe scope and spirit of the present invention.

It should also be noted that while preferred embodiments of the presentinvention have been described in connection with gunshot locationsystems, the techniques for providing precision locations from an arrayof GPS based sensor can be applied to other types of systems, such asthose monitoring environmental conditions, geophysical data collection,and the like.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned above as well as those inherenttherein. While presently preferred embodiments have been described forpurposes of this disclosure, numerous changes and modifications will beapparent to those skilled in the art. Such changes and modifications areencompassed within the scope and spirit of this invention.

1. A gunshot detection system comprising: an array of sensors, at leastone sensor of said array of sensors comprises: a microphone forreceiving acoustic signals; a processor for processing received acousticsignals to detect an acoustic event; a first commercial GPS receiver fordetermining a sensor position; and a first network interface; adifferential node at a known location, said differential nodecomprising: a second GPS receiver for determining a GPS position; and asecond network interface; and a server comprising: a CPU; and a thirdnetwork interface in communication with said first and second networkinterfaces such that said one or more sensors can communicate with saidhost node and said differential node can communicate with said hostnode, wherein said at least one sensor communicates said sensor positionto said server and said differential node communicates said GPS positionto said server and wherein when said acoustic event is detected, said atleast one sensor communicates event information to said server and saidserver computes a precision position for said at least one sensor basedon said sensor position and said GPS position.
 2. The gunshot detectionsystem of claim 1 wherein said server and said differential node arecollocated.
 3. The gunshot detection system of claim 1 wherein saidknown location is a surveyed location.
 4. The gunshot detection systemof claim 1 wherein said second GPS comprises a military GPS and saidknown position is provided by said military GPS.
 5. The gunshotdetection system of claim 1 wherein said server is configured to receiveWAAS information from either said differential node or said at least onesensor and, upon receiving said WAAS information, said servercommunicates said WAAS information through said third network interface.6. The gunshot detection system of claim 5 wherein said differentialnode is a second sensor of said array of sensors.
 7. The gunshotdetection system of claim 1 wherein said sensor position includes aplurality of pseudoranges.
 8. The gunshot detection system of claim 1wherein said GPS position includes a plurality of pseudoranges.
 9. In asystem having an array of commercial GPS based sensors, a method fordetermining a precision position for at least one sensor of said sensorarray including the steps of: (a) receiving a sensor position from saidat least one sensor; (b) receiving a GPS position from a commercial GPSat a known location; (c) calculating a position difference between saidknown location and said GPS location; and (d) calculating a precisionposition by adjusting said sensor position with said positiondifference.
 10. The method for determining a precision position of claim9 wherein step (b) includes the substeps of: (b)(i) receiving a firstposition from a military GPS to determine a known location; and (b)(ii)receiving a GPS position from a commercial GPS at said known location.11. The method for determining a precision position of claim 9 whereinstep (b) includes the substeps of: (b)(i) surveying a first position todetermine a known location; and (b)(ii) receiving a GPS position from acommercial GPS at said known location.
 12. A method for determining aprecision position for at least one sensor of a sensor array whereinWAAS information is available from a commercial GPS located proximatesaid sensor array, the method including the steps of: (a) receiving WAASinformation from said commercial GPS; (b) transmitting said WAASinformation to said at least one sensor; (c) receiving said WAASinformation at said at least one sensor; (d) calculating a precisionposition for said at least one sensor by adjusting a local positionprovided by a local GPS at said at least one sensor with said WAASinformation.